calculation of PDS-piecewise direct standardization, a method which uses the spectra acquired with a spectrometer A to estimate the spectra that would have been obtained with a spectrometer B
[model_pds] = pds_calc(x_B,x_A,k)
spectra of n samples acquired on a spectrometer B; a div or a matrix of dimensions (n x q)
spectra of the same n samples than previously, in the same order, acquired on a spectrometer A; a div or a matrix of same dimension as x_B
the length of a window; an odd number
the pds model; a div structure
model_pds.p is of dimensions (q x q)